Conductive/insulating graded GaAs bulk material

ABSTRACT

A composition of matter comprising a bulk material of uniform composition having first and second spaced apart surface regions and a dopant in the bulk material of progressively increasing concentration in a direction from the first to said second surface regions providing an interface intermediate the first and second surface regions wherein the portion of the bulk material on one side of the interface is electrically conductive and the portion of the bulk material on the other side of the interface is relatively electrically insulative. The bulk material is one of Ge, Si, group II-VI compounds and group III-V compounds and preferably GaAs or Gap. The dopant is a shallow donor for the bulk material involved and for GaAs and GaP is Se, Te or S. The ratio of the resistivity of the portion of the bulk material on one side of the interface to the portion of the bulk material on the other side of the interface is at least about 1:10 7 .

CROSS REFERENCE TO PRIOR APPLICATIONS

This is a continuation of application Ser. No. 7/977,388 filed Nov. 17,1992 and a continuation-in-part of Ser. No. 07/748,602, filed Aug. 22,1991, the contents of which are incorporated herein by reference nowabandoned.

BACKGROUND OF TEE INVENTION

1. Field of the Invention

This invention relates to novel bulk materials doped in a manner tocause rapid change from an electrically conductive portion to anelectrically insulative portion resulting from a small change in thedoping profile.

2. Brief Description of the Prior Art

No prior art is known wherein a bulk material changes rapidly fromelectrically insulating to electrically conductive or vice versa as aresult of a small change in doping profile thereof.

It is known from the above noted copending application that Si, Ge,Group III-V and Group II-VI compounds and particularly gallium arsenide(GaAs) and gallium phosphide (GaP), if appropriately fabricated (i.e,grown and doped) provide excellent electrooptical (EO) window and domecandidate materials for infrared (1 to 14 μm wavelength) transparency.Both GaAs and GaP can be doped with a shallow donor in an amount fromabout 5×10¹⁵ atoms/cc to about 2×10¹⁶ atoms/cc with a preferred amountof 8×10¹⁵ atoms/cc to render the materials conductive with resistivitiesup to about 0.1 ohm-cm. The above noted copending application discussesGaAs with desired resistivity of from about 0.07 to about 10 ohm-cm witha preferred resistivity of about 0.1 ohm-cm and an electron mobility ofgreater than about 3000 cm² /volt-second and preferably about 5000 cm²/volt-second. If the amount of carbon in the melt is greater than 1×10⁷atoms/cc, then an increased amount of Se must be used, such as about5×10¹⁶ atoms/cc with inferior results. The preferred shallow donor forGaAs or GaP is selenium (Se), though tellurium (Te) and sulfur (S) canalso be used with inferior results (less uniformity) since Se, whichsegregates less during growth, fits into the lattice structure of GaAsand GaP better than do Te or S. The shallow donor used, of course, mustbe matched to the lattice structure of the material with which it willbe associated.

A shallow donor is one wherein the amount of energy required to ionizeor remove an electron from the outer or valence band is extremely low,the energy at room temperature being more than sufficient to pull theelectron off, creating a conduction electron. The conduction electronshave high mobility and result in free-carrier absorption in thematerial. The high mobility of the conductive electrons creates a largedependence of the free-carrier absorption on wavelength whereby, atlonger wavelengths (e.g. about 10 GHz range), the material doped withthe shallow donor becomes very highly absorbing with effectively noabsorption at lower wavelengths (i.e., the infrared or 1 to 14 micronrange).

Another reason for selecting shallow donors is that they are fullyionized at room temperature. At room temperature, the shallow donorswill have contributed all of their donor electrons to the conductionband. Therefore, if the temperature of the material changes, there islittle change in the physical properties of the material. Thisconductivity can be controlled so that the GaAs and GaP remain infraredtransparent in the IR frequency range while offering substantialelectromagnetic interference (EMI) and electro-magnetic pulse(EMP)(EMI/EMP) protection or shielding and being opaque to frequenciesoutside the infrared frequency range of interest (i.e., 1 to 14microns). This protection is due to the coupling of the EMI/EMP to thefree carriers in the GaAs or GaP. This coupling causes reflection and,to a much greater extent, absorption of the EMI/EMP. Specifically,n-type GaAs with a resistivity of about 0.1 ohm-cm and high electronmobility greater than 5000 cm² /volt-second has been fabricated bydoping with selenium, though tellurium (Te) and sulfur (S) can also beused, resulting in a material with measured optical and EMI/EMPproperties as follows:

n-Type GaAs EMI/EMP Window/Dome Properties

    ______________________________________                                        Optical Absorption Coefficients (cm.sup.-1)                                                      Attentuation @ 15 GHz (dB)                                 ______________________________________                                        <0.02              >60                                                        ______________________________________                                    

This highly transparent and EMI/EMP shielding GaAs has no modulationtransfer function (MTF) losses. Similar properties are achieved in GaP.These materials provide the solution to the problem of infrared windowsand domes with inadequate EMI/EMP protection for EO systems.

The crystal growth as described in the above noted copending applicationrequires that such growth start at the bottom and preferably at thecenter of the melt and gradually continue outwardly and upwardly towardthe sides of a tapered vessel and then only upwardly when the crystalgrowth has reached the sides of the vessel. Such crystal growth causes alarge portion of the impurities in the melt, including a large portionof the shallow donor, to move upwardly in the melt. The result is thatthe shallow donor doping concentration increases in an upward directionor direction of crystal formation because the donor concentration in themelt continually increases as more of the melt is crystallized.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a materialwhich demonstrates an abrupt change from electrically insulating toelectrically conducting due to change in the doping profile therein.This change can be, for example in GaAs, from a resistivity in excess of10⁷ ohm-cm to a resistivity of less than 10 ohm-cm over a distance ofless than one mm. The abrupt transition, for example, takes place at afree carrier concentration in the 10¹⁴ atoms/cc range. This abrupttransition takes place, for example, at approximately 1×10¹⁶ atoms/cc ofshallow donor with a deep donor concentration of approximately 1×10¹⁶atoms/cc and a shallow acceptor concentration of approximately 1×10¹⁶atoms/cc. The doped bulk material is one of germanium, silicon, a groupII-VI or group Ill-V compound and preferably gallium arsenide or galliumphosphide. The bulk material is doped with a shallow donor for the bulkmaterial in question which is to be made n-type conductive. Selenium,tellurium and sulfur are shallow donors which can be used with galliumarsenide and gallium phosphide with selenium being the preferred shallowdonor.

The crystal is grown from the bottom and preferably central portion ofthe bottom in a direction upwardly and to the side as described in theabove noted copending application so that the doping distribution,acting in concert with the native impurities and defects of the bulkmaterial, render the bulk material essentially a two layer material, onelayer being highly electrically conductive with a sharp interface andtransition to a high resistivity or Insulating layer. Such materialshave enabling application as EMI shields and electronic devices, such asIR windows and domes and MMIC devices. Electrical devices, such ascapacitors and the like can also be fabricated from the bulk material ofthe present invention by adding an electrically conductive layer overthe insulating portion thereof. The ability to change the thickness ofthe two layers with precision, such as by abrading or the like of thebulk material, allows for modulation of the reflection from one side ofthe bulk material while providing high attenuation and reflection fromthe other side of the material in the GHz range. One side is metallic,i.e., high reflection, and the other side is an insulator backed by theconductive layer creating two interfaces where the phase of the incidentelectromagnetic energy is modulated by its reflection from the outermostinsulating surface interfering with the reflection from theinsulating/conduction layer interface.

Briefly, assuming that the bulk material is gallium arsenide (it beingunderstood that any of the above-mentioned other bulk materials can besubstituted therefor), a melt of gallium arsenide with selenium ofapproximately 3×10¹⁶ atoms/cc therein (it being understood that dopantsother than selenium may be required for bulk materials other than GaAs)and a boric oxide layer thereover is crystallized by freezing from thebottom up as described in the above noted copending application. Thebottom up growth distributes the dopant with continually greater dopantconcentration in the upward direction. This is the result of the naturalsegregation of the dopant. Both the literature and experiment indicate asegregation coefficient of 0.35 for selenium in GaAs. Calculating thesegregation or selenium atomic concentration through the thickness of alarge 12"×12" vertical gradient freeze crystals of approximately 1×10¹⁶atoms/cc on the insulating side to 1×10¹⁷ atoms/cc on the opposite sideand the resulting electrical resistivity, assuming a deep donorconcentration of 2×10¹⁶ atoms/cc and a shallow acceptor concentration of2×10¹⁶ atoms/cc (e.g. carbon impurity), shows a very sharp resistivitygradient of seven orders of magnitude (FIG. 1). The sharp resistivity orelectric conductivity gradient in the Gaps and the fact that one side issemi-insulating (≧10⁷ ohm-cm) and the other side is highly conductive(about 1 ohm-cm) results in a unique sandwich or apparent two layeredmaterial. The resulting interaction of electromagnetic energy with thisGaAs represents an extremely effective and novel electromagneticinterference (EMI) shielding device with concomitant particularapplication in the form of an infrared (IR) transparent window or domefor electro-optic sensors on aircraft or missiles. This absorptionproperty affects electromagnetic energy in the 10 to 100 GHz range onthe order of a 60 dB or more one way attenuation and about 30 dB or moreattenuation below 10 GHz into the MHz region.

The other component to this attenuation, in addition to the absorption,is reflection. On the highly conductive surface, the reflectivity issimilar to that of a metal, ≧99%. To eliminate reflections and stillmaintain the high absorption and therefore the high EMI shielding in theGHz range, the other or insulating side of this GaAs is of importance.The steep gradient of the resistivity effectively results in twoessentially distinct layers in the GaAs. The high resistivity side andthe sharp transition therefrom to the highly conductive, low resistivityside results in greatly different reflection characteristics than themetallic-like reflectivity at the conductive side. By grinding andpolishing the high resistivity side to, for example, quarter-wavethickness for the electromagnetic energy of interest (e.g. 10 GHz), thereflection can be eliminated or minimized by cancellation of energyreflected from the metallic-like surface by energy of like frequencyreflected from the interface after travel of a quarter wave length ineach direction through the high resistivity portion. This results in ahighly absorbing EMI shield providing IR windows or domes with verylittle or no reflection of that EM energy. This effect can be producedin Si, Ge, GaAs, GaP, etc.

The effects set forth above are a direct consequence of the novel growthand doping process specifically designed to produce the resistivitygradient through the thickness of the GaAs. The transition is very sharpfrom a resistivity of about 1×10⁷ to a resistivity of about 10 ohm-cm.

While the attenuation is essentially the same in either direction (seeFIGS. 1A and 1B) and the reflectivity in the GHz frequency region of theconductive side is very high (>99%), the reflectivity of thenon-conductive (high resistivity) side is significantly lower due to twoeffects. First, the high resistivity results in a small complexdielectric function (i.e., lower absorption, lower imaginary part of thedielectric function), therefore representing a smaller impedancemismatch at the surface between the GaAs and air. This is similar tocommon optics where the so-called "Fresnel" reflection takes place at aninterface. The reflection amplitude is dependent upon the index ofrefraction or dielectric constant difference between the two materialsat the surface (i.e., GaAs and air). Second, the sharp change inresistivity within the bulk of the GaAs and the thickness of the highresistivity layer before this sharp change will modulate thereflectivity due to interference effects. This is again similar tocommon optics where surface reflections are commonly controlled byimpedance matching layers, usually called anti-reflection (AR) layers.Thus, by grinding the high resistivity side to a specified thickness(e.g., 1/4 wavelength), the high resistivity side will act as a quarterwave AR layer at the specified thickness. For example, if the dielectricfunction of the high resistivity side is E_(R), the quarter-wavethickness of the high resistivity side is approximately λ/4(E_(R))⁻²,where E_(R) is approximately 12 for GaAs and λ is approximately 3 cm for10 GHz. Therefore, 3 cm/(4×12⁻²)=2.5 mm. This approximation requiresequal intensity reflection from the front surface and the interface,which is not necessarily true and this is only an approximation. Exactcalculations of the dielectric function of the two sides of the GaAs arerequired to obtain exact predictions of these electromagnetic effects.

In summary, the unique GaAs described in the above-mentioned copendingapplication is even more unique than previously described. Theconsequences of the designed growth process therein result inessentially a two layered GaAs material, one layer being conductive,with a sharp interface to the other high resistivity layer. This novelbulk GaAs material results in highly effective electromagneticinterference (EMI) shielding material which is particularly effective inthe GHz frequency range. Another novel aspect of this GaAs is itsability to modulate and thereby minimize the reflection on one side.This is of great interest to applications requiring a material with IRtransmission and very high GHz absorption and lower GHz reflection. TheGaAs is close to "transparent metal" and with the ability to lower thereflectivity of one side well below that of the near-metallicreflectivity of the other side. It is a transparent non-reflectivemetal. Beside EMI shielding, the novel GaAs may also be used tofabricate electronic devices based upon GaAs, such as MMIC and othermicrowave devices. Various GaAs ICs or other devices can benefit fromthis novel GaAs since it possesses both a high resistivity (electricallyinsulating) and an electrically conductive side. This Gabs can be usedas a substrate for these type devices. Other novel devices can be basedupon this GaAs since it permits propagation and resonance of GHz ormicrowave frequencies therethrough.

BRIEF DESCRIPTIONS TEE DRAWING

FIGS. 1A and 1B are graphs showing GaAs response in GHz.

FIG. 1C is a graph showing the change in resistivity with distance fromthe bottom of a GaAs bulk member formed in accordance with the presentinvention; (This figure or another figure should show the difference indoping level along the curve);

FIG. 2 is a vertical gradient growth apparatus for forming galliumarsenide in accordance with the present invention;

FIG. 3 is a side view of heating element surrounding the crucible ofFIG. 2; and

FIG. 4 is a top view of the heating elements of the growth apparatus ofFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 2, GaAs of optical quality with a uniform andcontrollable resistivity and with large dimension (e.g., 12 inches by 12inches) is produced. The process uses a pressure chamber 1 havingtherein a graphite boat or crucible 3 with tapered sides and bottom intowhich gallium 5, shallow donor selenium (Se) in an amount of 2×10¹⁶atoms/cc and boron oxide (B₂ O₃) glass encapsulant 7 are loaded. Arsenicwhich has been loaded into an injector 9 made of fused quartz is loweredto the injection position to inject the arsenic into the crucible 3after opening of the isolation valve 13 to provide a stoichiometric meltof GaAs with the small amount of Se therein in an argon atmosphere at apressure of 20 to 30 psig.

The chamber 1 is maintained at the pressure of 20 to 30 psig of argon.The furnace 11 and therefore the Ga 5 is heated to a uniform temperatureabove the melting point of GaAs (1238° C.). In order for the Asinjection to proceed to completion, the gallium 5 is held from 1 to 15°C. above 1238° C. Too hot or too cold gallium will result in loss ofarsenic due to the arsenic vapor pressure overcoming the pressure on theencapsulant 7 or the arsenic exceeding the solubility limit if thegallium is below 1238° C. Stoichiometry is obtained by maintaining therequired exact amounts of gallium and arsenic with a small 0.5% or lessexcess of arsenic to allow for residual arsenic in the injector 9 andsmall losses from the melt. The control of the exact amount of arsenicto less than 0.5% excess As than needed for stoichiometry is one of thekey controls of very low absorption at 1.06 microns.

A multi-zone furnace 11 is used to control the melt and crystal growth.A typical furnace for use in conjunction with the present invention isshown in FIGS. 3 and 4 wherein the graphite boat 3 is shown supported ona boat support 21. A carbon cloth 23, preferably of graphite having athickness of 20 mils rests over the entire bottom of the boat 3. Theboat 3 is surrounded by heating elements of the furnace 11 (FIG. 2)which include, as shown in FIGS. 3 and 4, a single element A beneath thecenter of the boat, four heating elements B in a circle spaced outwardlyfrom element A, eight elements C in a circle spaced outwardly fromelements B, four elements D in a circle spaced outwardly from elementsC, sixteen side elements E in two levels spaced in a circle about theboat and two top elements F disposed over the boat.

Once the arsenic has been sublimed into the gallium, forming liquid GaAsencapsulated by the boron oxide glass encapsulant 7 under pressure of 20to 30 psig in the vessel 3, the GaAs crystal is then frozen or grownfrom the bottom up.

The growth proceeds with the bottom center zone of the furnace 11lowering its temperature below the melting point of GaAs, 1238° C. Thiscauses the GaAs melt in the boat 3 to nucleate and begin to grow fromthe bottom center in a primarily outward and upward direction to thewalls of the boat 3 and then primarily in an upward direction undercontrol of the various sections of the furnace 11 as discussedhereinbelow. The boat 3 is tapered on the bottom and sides aid inbeginning the crystal growth at the bottom center and, ultimately, therelease of the final crystal from the boat. The temperature of the otherbottom zones, designed in a radially symmetric pattern, are loweredbelow 1238° C., creating a radially symmetric thermal gradient asgenerally follows with reference to FIGS. 3 and 4:

    __________________________________________________________________________    Time after As                                                                 Injection                                                                            1 hr. 2 hrs.                                                                             3 hrs.                                                                             4 hrs.                                                                             6 hrs.                                                                             9 hrs.                                       __________________________________________________________________________    A      1230° C.                                                                     1225° C.                                                                    1220° C.                                                                    1215° C.                                                                    1200° C.                                                                    1200° C.                              B      1232° C.                                                                     1227° C.                                                                    1222° C.                                                                    1217° C.                                                                    1200° C.                                                                    1200° C.                              C      1236° C.                                                                     1235° C.                                                                    1225° C.                                                                    1220° C.                                                                    1205° C.                                                                    1200° C.                              D      1236° C.                                                                     1235° C.                                                                    1225° C.                                                                    1220° C.                                                                    1205° C.                                                                    1200° C.                              Sides  1245° C.                                                                     1245° C.                                                                    1245° C.                                                                    1240° C.                                                                    1230° C.                                                                    1200° C.                              Top    1245° C.                                                                     1245° C.                                                                    1245° C.                                                                    1245° C.                                                                    1240° C.                                                                    1200° C.                              __________________________________________________________________________

This results in the growth of the crystal from the center up and thecenter out to the edges. Then the side zone temperatures are lowered attime 4 hours while the top zones are held at 1245° C. This allows aplanar vertical interface to advance up through the melt, growing alarge flat plate of GaAs. Eventually the temperatures of the side andtop zones are lowered to allow completion of the crystal growth. Thevery small thermal gradients within the growing crystal (less than 20°C.) result in the stoichiometric, low defect, low dislocation GaAs, witha distribution of dopant through the thickness of the crystal. Thedoping is uniform across the crystal, which is essential for use as awindow or dome.

The crystal is annealed at 850° C., by lowering all furnace zones tothis temperature at a rate of about 2° C./minute and is held at thistemperature for 8 to 12 hours. As the temperatures in the zones are thenlowered at about 2° C./minute after annealing in the range of 550 to520° C. and preferably to 530° C., the pressure is vented from thechamber and evacuated with pumps to about -14.7 psig. This causes the B₂O₃ glass encapsulant 7 to reboil (dissolved gas bubbles out), resultingin a foaming glass which becomes solid as the furnace lowers intemperature below 400° C. This foamed B₂ O₃ is too brittle to fracturethe crystal as the temperature is lowered to room temperature (about 20to 25° C.). Once cooled, the crucible 3 with the GaAs crystal and thefoamed B₂ O₃ thereover are placed in alcohol (preferably methanol,ethanol, propanol or isopropanol) and then the B₂ O₃ is completelydissolved in water, releasing the GaAs crystal from the crucible. TheGaAs crystal can now be fabricated into a window.

Though the invention has been described with respect to specificpreferred embodiments thereof, many variations and modifications willimmediately become apparent to those skilled in the art. It is thereforethe intention that the appended claims be interpreted as broadly aspossible in view of the prior art to include all such variations andmodifications.

I claim:
 1. An EMI shield for shielding EMI radiations of a knownfrequency comprising:(a) a bulk semiconductor material of uniformcomposition having first and second spaced apart surface regions, (b)said bulk semiconductor material being doped with a dopant withprogressively increasing dopant concentration in a direction from saidfirst to said second surface regions, (c) said doped semiconductormaterial having an interface intermediate said first and second surfaceregions, (d) said doped semiconductor material having an interfaceintermediate said first and second surface regions said dopedsemiconductor material demonstrating an abrupt change from relativelyhighly electrically conductive on one side of said interface torelatively highly electrically insulative on the opposing side of saidinterface in the region of said bulk semiconductor material immediatelyadjacent said interface, (e) the thickness of the electricallyinsulative side of said bulk semiconductor material being one quarter ofa wave length of said known frequency.
 2. The shield of claim 1 whereinsaid bulk semiconductor material is taken from the class consisting ofGaAs and GaP.
 3. The shield of claim 2 wherein said dopant is taken fromthe class consisting of Se, Te and S.
 4. The shield of claim 1 whereinthe ratio of the resistivity of said portion of said bulk semiconductormaterial immediately adjacent one side of said interface to the portionof said bulk semiconductor material immediately adjacent the other sideof said interface is at least about 1:10⁷.
 5. The shield of claim 2wherein the ratio of the resistivity of said portion of said bulksemiconductor material immediately adjacent one side of said interfaceto the portion of said bulk semiconductor material immediately adjacentthe other side of said interface is at least about 1:10⁷.
 6. The shieldof claim 3 wherein the ratio of the resistivity of said portion of saidbulk semiconductor material immediately adjacent one side of saidinterface to the portion of said bulk semiconductor material immediatelyadjacent the other side of said interface is at least about 1:10⁷. 7.The shield of claim 4 wherein said abrupt change takes place over adistance of less than one millimeter.
 8. The shield of claim 5 whereinsaid abrupt change takes place over a distance of less than onemillimeter.
 9. The shield of claim 6 wherein said abrupt change takesplace over a distance of less than one millimeter.